Noise has long been a problem in electronic applications. Many techniques have been developed to discriminate between a desired signal and noise.
In most cases the noise which is to be suppressed differs in its temporal characteristics from the desired signal. For instance, speech signals tend to contain bursts of information whereas a noise signal, such as low frequency noise from an air conditioning unit, tends to have the same anplitude over a longer period of time.
Simple noise suppression systems have been designed to integrate the incoming signal, a desired signal plus noise, over a given period of time and compare the integrated value with a threshold. Above the threshold it is assumed there is noise and a filter kicks in to remove the noise.
This simple system has a number of drawbacks. The integrator is amplitude sensitive. A large amplitude short duration signal, normally containing non-noise information, can be over the threshold when integrated. This will generate a false trigger of the filter. As well, when the filter kicks in some of the signal strength will be lost. The difference in strength between a filtered and non-filtered signal will be evident at the output. In many instances this is undesirable, especially when dealing with audio signals.
It is an object of this invention to overcome one or more of these problems.